1. Field of the Invention
The present invention relates to a means and method of optimizing the efficiency of combustion devices such as furnaces, boilers, ovens, stoves, and the like, and in particular, relates to an automatic control means and method for determining and holding the combustion device at an optimum efficiency level.
2. Problems in the Art
Until only relatively recently, fossil or fossil-based fuels were in ready supply and thus were available at low or nominal cost. Combustion devices such as furnaces, boilers, ovens, stoves and the like, thus were economical to run regardless of efficiency.
The tremendous industrial expansion and corresponding depletion of world supplies of fossil fuels has made efficiency for combustion devices a critical aspect in their use. Whether it be large scale heating plant furnaces or boilers, or household fossil fuel burning ovens or stoves, the very much higher cost and increasing scarcity of fossil fuels has required introducion of methods to increase efficiency to decrease the amount of fuel used per unit of output.
Whereas the actual construction and operating structure and elements of the combustion devices have been extensively redesigned or modified to produce significant increases in efficiency, a major problem still exists in controlling the basic combustion process. Combustion is a product of combustible fuel with oxygen.
In combustion devices, it is required that an excess amount of air (containing the needed oxygen), over the stoichiometric amount needed for combustion, be utilized for the following reasons. If the combustion process has insufficient air, first, there would be incomplete combustion thereby wasting fuel, secondly, there would be slagging problems because of incomplete combustion, and thirdly and most significantly, the risk of overheating, damage to the device, and even explosion is dramatically increased. Therefore, all combustion processes make sure that excess air is always available.
However, too much excess air decreases the efficiency of the furnace or combustion device by increasing the heat loss out the stack. However, the danger of explosion is not there.
Therefore, there is a real need to develop a system by which the level of excess air for a given fuel supply is maintained at an "optimum" level where there is a compromise between combustible losses and heat losses from heated excess air leaving the furnace.
Conventionally, targets for excess air levels are created on the basis of imperical knowledge of the combustion process. Combustion engineers therefore calculate the amount of air input to the device based upon the known properties of the fuel and the output of the furnace. While this gives a general estimate, there is much room for error in that the properties of fuel change significantly during combustion, the calculations based on output and properties of the fuel are mere approximations, and in many of these combustion devices, the load (firing rate) changes over time. The combustion engineer can attempt to diminish these variables by monitoring the excess air level generally using a O.sub.2 (Oxygen) analyzer. By closely limiting the amount of excess air, as indicated by the gas analyzer, and diligently keeping the air at approximately the most efficient level, significant efficiency improvement can be realized. For example, in a large scale coal burning furnace, by reducing the excess air to achieve optimal combustion without danger, an increase in efficiency of 1% could mean millions of dollars per year.
Automatic control of the excess air level is implemented by the economy of the microprocessor art. Examples of the use of automatic furnace combustion controls can be found in the following U.S. Pat. Nos: 4,045,292; 4,238,185; 4,330,261; 4,421,473; 4,439,138; 4,449,918; and 4,474,121. In attempting to automatically control the combustion parameters to optimize efficiency, some of these methods and apparatus attempt to minimize excess air by looking to the combustion by-products for indication of how much air needs to be input. The problems exhibited by such methods are that the equipment needed to derive the required information about the gases are inherently unreliable, are subject to significant error, and are generally expensive to purchase, install, and maintain. Additionally, they require periodic calibration and if disabled or miscalibrated, the system simply does not function according to its intended advantage. Also, combustion systems are always subject to leakage as to air and the gaseous components and therefore the gas readings may be misleading for that reason. Note U.S. Pat. No. 4,449,918.
The combustion engineer can check his control of the excess air level by checking the efficiency of his furnace by energy accounting, either by using the losses method (as is known in the art) or by careful measurement of the fuel input and output of the furnace. The engineer could even try different excess air levels and re-test the efficiency for a gain. Howver, these methods are tedious, subject to error and not used for day-to-day operation of a furnace.
It is therefore a principal object of the invention to provide a means and method for optimizing efficiency of a combustion device which improves over or solves the deficiencies in the art.
A further object of the invention is to provide a means and method of optimizing efficiency of a combustion device which determines optimal efficiency excess air levels by monitoring generally the output of the combustion device during testing.
Another object of the invention is to provide a means and method of optimizing efficiency of a combustion device which continually tests for optimal combustion efficiency and insures that a minimum required amount of excess air is always available.
Another object of the invention is to provide a means and method of optimizing efficiency of a combustion device which determines whether to increase or decrease air input by evaluating changes in rate of output of the combustion device over time.
A further object of the invention is to provide a means and method of optimizing efficiency of a combustion device which can be utilized in combustion devices having variable fuel and air inputs or fixed fuel input.
A further object of the invention is to provide a means and method of optimizing efficiency of a combustion device which utilizes a retrofittable control circuitry operable with existing combustion device hardware to achieve its result.
Another object of the invention is to provide a means and method of optimizing efficiency of a combustion device which can be adjusted to have various testing periods, various increments of increase or decrease in excess air during testing, and various reset capabilities.
A further object of this invention is to provide a means and method for optimizing efficiency of a combustion device which can utilize either microprocessor apparatus or analog apparatus to accomplish its functions.
A further object of this invention is to provide a means and method of optimizing efficiency of a combustion device which is economical, durable, accurate and efficient.
These and other objects, features and advantages will become apparent with reference to the accompanying specification and drawings.